A conventional backhoe includes a frame which may be mounted on the rear of a tractor or like implement. The frame supports a boom assembly for pivotal movement with respect to the frame about a vertical axis. The boom assembly typically includes a bucket or other material-handling device for work operations, with articulation of the boom assembly by the backhoe operator providing material-handling in the desired fashion.
It is desired that the work operations be efficiently performed with the backhoe, and thus it is preferable to perform such operations in the shortest amount of time. Typically, such operations include a cycle of filling the bucket with material at one location, pivoting the boom assembly, dumping the material from the bucket at another location, and then pivoting the boom assembly back to the original location to end the cycle. The cycle is usually repeated many times.
In order to perform these operations as quickly as possible, the backhoe operator typically swings or pivots the boom assembly as rapidly as possible. In early backhoe designs, this frequently resulted in the boom assembly being forced hard against mechanical travel stops which limited the pivoting movement of the boom assembly. While this practice helped reduce the pivoting time during the work cycle, the practice was found to be detrimental to the structural components. On the other hand, if the operator exercised greater care during operation by manipulating the boom assembly swing control to slow the assembly before it reached the travel stop, then shock loading of the machine's components was reduced, but the extra care exercised by the operator increased the time for completing each cycle of backhoe operation.
In order to alleviate the problems caused by shock loading of the backhoe as the boom assembly is pivoted against a mechanical travel stop, various arrangements have been employed or proposed in the past to provide hydraulic cushioning of the boom assembly as it approaches the end of its arc of travel. One such prior art arrangement employs a "stinger" design which includes a projection carried by the piston of each hydraulic motor provided for swinging the backhoe boom assembly. As the boom assembly of the backhoe approaches the ends of its arc of travel, the stinger of one of the hydraulic motors acts to restrict the flow of hydraulic fluid from that motor, thus providing hydraulic cushioning of the boom assembly. While combination stinger/orifice cushioning arrangements have been widely used, their fabrication and maintenance have proven to be relatively expensive.
The U.S. Pat. No. 4,500,250 describes in detail an improved backhoe boom swing mechanism which includes a linkage assembly connecting the boom assembly with a sequencing spool valve. The spool valve is selectively operated as the boom assembly is pivoted through its arc of travel to direct the hydraulic motor discharge flow through a hydraulic cushioning circuit.
While this arrangement is a significant improvement and functions well, it would be desirable to provide a further improved arrangment that completely avoids the use of exposed mechanical devices, such as linkages and the like, that are vulnerable to being damaged as a result of operator error causing such devices to impact against external objects or as the result of external objects being moved against such devices. Further, it would be desirable to provide a further improved mechanism for controlling boom assembly swing that would be less susceptible to mechanical wear.
Another problem of conventional backhoe design relates to the use of double-acting piston-cylinder type hydraulic motors to pivot the boom assembly. Spatial limitations are a major consideration in positioning the hydraulic motors of the swing mechanism. The motors are usually positioned generally adjacent each other, and are pivotally interconnected between the frame of the backhoe and the boom assembly. In order to obtain the desired range of travel for the boom assembly (approximately 180 degrees), this configuration of the conventional swing mechanism results in one or the other of the hydraulic motors moving through a fully extended position when the boom assembly is moved beyond either end of a central range of travel of approximately 90 degrees. This fully extended position is frequently referred to as the "center position" for that hydraulic motor. As the boom assembly is moved toward the ends of its arc of travel, one of the two hydraulic motors moves through its fully extended center position and goes "overcenter", and then begins to contract. Each hydraulic motor of the swing mechanism moves through its center position whenever its centerline (i.e., line of action) intersects the vertical swinging axis of the boom assembly (i.e., when the axes of the pivot connections at each end of the hydraulic motor are coplanar with the main pivot axis of the boom assembly).
As one of the hydraulic motors of such a conventional swing mechanism moves to and through its center position, the moment arm through which it acts upon the boom assembly approaches zero at the center position, and then reverses 180.degree. so that the overcenter hydraulic motor then exerts a negative torque on the boom assembly. Since the other (non-overcenter) hydraulic motor of the swing mechanism acts through a much greater moment arm than the overcenter motor, the boom assembly continues to move through its arc of travel. However, since the overcenter hydraulic motor exerts a negative torque on the boom assembly, the non-overcenter hydraulic motor must work to overcome this negative torque as it provides the primary force for pivoting the boom assembly.
The negative torque created by the overcenter one of the hydraulic motors is particularly a problem when the swinging movement of the boom assembly away from one of its travel stops is initiated, since inertial forces must be overcome. In this regard, a hydraulic swing mechanism for the boom assembly which includes an arrangement for redirecting the flow of hydraulic fluid to the hydraulic motors during swinging movement of the boom assembly so that the hydraulic motors are not, in essence, acting against each other, would provide a more efficient swing mechanism with improved control for the backhoe operator.
The special control linkage of the mechanism disclosed in the above-discussed U.S. Pat. No. 4,500,250 operates to actuate a multi-position spool valve to reverse direction of the hydraulic motor flow relative to one of the hydraulic motors when that one motor goes overcenter. Although this provides a significant improvement in the torque characteristics of the system, it would be desirable to provide an even further improved control mechanism that would not require an external control linkage that is subject to damage from accidental impact and mechanical wear. Further, it would be desirable to provide an even further improved control mechanism in which the hydraulic motor torque control could be efficiently combined in one hydraulic system with the devices for cushioning the boom assembly at the ends of its arc of travel.